Question

Given a map, I need to retrieve and operate two immediately stored items.
To me, working on a vector is litter easier since I can do “iter + 1” or “iter – 1”.
While for map, I am out of luck.

For example, I give a simple example as follows:
Note: in my real application, I don’t simply subtract those numbers.

int main ()
{
    map<char,int> mymap;
    map<char,int>::iterator it;

    mymap['b'] = 100;
    mymap['a'] = 200;
    mymap['c'] = 300;

    // show content:
    map<char,int>::iterator firstItem  = mymap.begin();
    map<char,int>::iterator secondItem = ++mymap.begin();

    for ( ; secondItem != mymap.end(); ++firstItem, ++secondItem )
        cout << secondItem->second - firstItem->second << endl;

    return 0;
}

Question> Is there a better solution for this?

Thank you

Answer 1

Instead of incrementing both iterators in the loop control (incrementing is a bit slow), just assign firstItem = secondItem then increment secondItem.